cmd/lncli: add fwdinghistory support for relative times

- introduces new parser in `cmd/lncli/arg_parse.go`
- converts start_time and end_time flags to strings
- adds default value for end_time

cmd/lncli/arg_parse.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"regexp"
+	"strconv"
+	"time"
+)
+
+// reTimeRange matches systemd.time-like short negative timeranges, e.g. "-200s".
+var reTimeRange = regexp.MustCompile(`^-\d{1,18}[s|m|h|d|w|M|y]$`)
+
+// secondsPer allows translating s(seconds), m(minutes), h(ours), d(ays),
+// w(eeks), M(onths) and y(ears) into corresponding seconds.
+var secondsPer = map[string]int64{
+	"s": 1,
+	"m": 60,
+	"h": 3600,
+	"d": 86400,
+	"w": 604800,
+	"M": 2630016,  // 30.44 days
+	"y": 31557600, // 365.25 days
+}
+
+// parseTime parses UNIX timestamps or short timeranges inspired by sytemd (when starting with "-"),
+// e.g. "-1M" for one month (30.44 days) ago.
+func parseTime(s string, base time.Time) (uint64, error) {
+	if reTimeRange.MatchString(s) {
+		last := len(s) - 1
+
+		d, err := strconv.ParseInt(s[1:last], 10, 64)
+		if err != nil {
+			return uint64(0), err
+		}
+
+		mul := secondsPer[string(s[last])]
+		return uint64(base.Unix() - d*mul), nil
+	}
+
+	return strconv.ParseUint(s, 10, 64)
+}